Preparations of chemically unmodified immunoglobulin which constitute plasma protein, and particularly preparations containing IgG as a main ingredient have been widely used for treatment and prophylaxis of various infectious diseases. Since .gamma.-globulin is readily polymerized in a dissolved state which causes side effects when administered intravenously, it has hitherto been formulated into freeze-dried preparations. The freeze-drying method is also believed to be the best form for storing the .gamma.-globulin from the standpoint of lability of chemically unmodified .gamma.-globulin having a complete molecular structure when dissolved. Accordingly, the importance of freeze-drying is generally recognized.
On the other hand, liquid preparations are advantageous over the freeze-dried preparations due to convenience of administration because liquid preparations are free of the need to be dissolved, for example, in injectable distilled water on use. However, the above-mentioned lability of .gamma.-globuloin has retarded the practical application of liquid preparations of .gamma.-globlulin.
A .gamma.-globulin liquid composition having a pH of from about 3.5 to 5 and an ionic strength of less than about 0.001 has recently been proposed as a stable liquid preparation as disclosed in U.S. Pat. No. 4,396,608 or European Patent Publication No. 73371A. However, such a strongly acidic liquid preparation is not always favorable when administered to a living body because the .gamma.-globulin is liable to aggregate (i.e., to polymerize) in body fluids maintained substantially neutral due to the buffering capacity of such body fluids.